The present invention relates to an apparatus for measuring the time between pulses transmitted by a sending transducer and received by receiving transducers with known transient behavior and for eliminating measuring results which are erroneous due to travel of the pulses in the transmission medium over propagation paths of different lengths.
In the reflected beam ranging art, pulse-shaped electromagnetic ot acoustic waves are transmitted and received again after being reflected. If the propagation speed in the transmission medium is known, the time between emission of the transmitted pulse and receipt of the received pulse is a measure of the length of the travel path. Conversely, if the distance is known, this time indicates the propagation speed.
Such time measurement is also effected to determine the flow speed of a flowing medium in that sound energy is transmitted in a "sing-around" method over a known measuring path oriented obliquely to the direction of flow and the repetition frequency of the transmitted or received pulses, or the time between successive pulses, is evaluated.
In order to be able to effect such time measurement, it is known, for example, to use a circuit arrangement, such as that of the acoustic flowmeter disclosed in British Pat. No. 776,526, in which each received pulse is checked in a threshold value detector to control triggering of the next transmitted pulse. If the oscillations of the received pulse exceed a given threshold, the next transmitted pulse is actuated and its repetition frequency is evaluated to measure the flow speed.
With the above measuring technique, the accuracy with which the time between pulses is measured is directly dependent on the precision with which the start of oscillations of a pulse can be determined. This time of receipt itself is not accessible to measurement because the envelope of the received pulse does not have an infinitely steep leading edge. Rather it is flattened by the transient behavior of the receiving transducer, and possibly of a subsequent amplifier circuit.
In the described circuit arrangement, the use of a threshold value results in performance of a time measurement at a point on the envelope leading edge which has a known position with respect to the time of receipt. However, the threshold value detector furnishes the wrong measuring results when the envelope of the received pulse has been deformed by interference.
Interference is produced, for example, by reflections of the wave energy at the boundaries of the transmission path since the beam angle of the emitted wave energy cannot be made as small as may be desired. Thus waves of different phase are superposed at the receiving end after traveling thereto on different propagation paths. In the most unfavorable case, the superposition leads to complete extinction, for example if the waves are received in phase opposition.
If the envelope is deformed in the region of its leading edge, the passage of the threshold is not an accurate measure for the start of the received pulse, so that reliable time measurement between two pulses cannot be made with a threshold value detector alone.